The fast growing field of brachytherapy utilizes radiation sources, such as beta emitters, gamma emitters and x-ray sources, for introduction into body cavities or organs, for radiotherapy. The object of radiotherapy is to produce biological damage of a controlled volume of tissue e.g., a tumor, while avoiding damaging adjacent normal tissues. In this method relatively high doses of x-rays, high energy charged particle beams or gamma rays emitted by radioactive isotopes irradiate diseased and normal tissues. The irradiated cells are damaged and die or are unable to reproduce.
Brachytherapy procedures include implanting radioactive sources in various shapes into specific organs or locations in the human body for prolonged irradiation treatment or introducing the radiation source via a suitable catheter or probe in cases where lower doses or short irradiation periods are required. Brachytherapy is applied in tumor therapy and usually involves the introduction of radioactive sources for radiation treatment into tumors or their vicinity in body cavities.
Another major evolving application in brachytherapy is the procedure of endovascular brachytherapy. Endovascular brachytherapy is intravascular irradiation treatment for prevention of restenosis, which occurs after angioplasty in coronary blood vessels by means of balloons and stents. The estimated number of angioplasty procedures in the U.S. is 500000 P/A with restenosis occurring in about 30% of cases.
Several radiation catheters, based on the use of radioactive sources such as beta- emitting .sup.32 P, .sup.80 Sr/.sup.90 Y, .sup.188 W/.sup.188 Re, beta+ .sup.48 V or gamma emitting .sup.192 Ir, are at various stages of development and implementation. These radioactive sources, in a variety of configurations, are introduced via special catheters into the blood vessels and the radioactive source is placed at the treatment position for a predetermined period for obtaining the proper irradiation dose. Typical doses required for the treatment are between 10-20 Gy (1,000-2,000 rem). Another method utilizing a radioactive source is the implantation of a radioactive stent based on the above radioactive isotopes.
In these methods there is limited ability to provide selective control of time dosage or radiation intensity. These methods include exposing healthy organs to dangerous radiation during the introduction of the radiation source and require handling radioactive materials which involves hazards to both the individual handling the radioactive materials and to the environment.
X-ray radiation is typically produced by high energy electrons generated and accelerated in a vacuum to impact on a metal target. The efficiency of x-ray generation is dependent on the electron beam current and on the acceleration voltage. An emitted x-ray spectrum is composed in part of discrete energies characteristic of transitions between bound electron energy levels of the target element. The spectrum also includes an x-ray energy continuum, known as bremsstrahlung, which is caused by deceleration of the beam of electrons as they pass near target nuclei.
Another method for the production of x-rays is by direct conversion of light into x-ray radiation. It is known that the interaction of light with a target can produce highly energetic x-rays when the power densities achieved are in the range of 10.sup.18- 10.sup.17 watt/cm.sup.2. With the development of femtosecond laser, such power densities are achievable with moderate size lasers (for example see C. Tillman et al, NIMS in Phys.Res. A394 (1997), 387-396 and references therein and U.S. Pat. No. 5,606,588 to Umstadter et al.). For example, a 100 femtosecond pulse of 1 mJ laser pulse focused down to a 3 micron spot, will reach this power density level. Medical applications of this method of x-ray generation are currently in development stage. These methods of x-ray generation have been considered for medical imaging [Herrlin K et al. Radiology (U.S.A.), vol189, no1, pp. 65-8, October 1993]. Another medical application of femtosecond lasers is in improved non-thermal ablation of neural or eye tissue for surgical purposes. [F. H. Loesel et al. Appl.Phys.B 66, 121-128 (1998). The development of compact table top models of femtosecond lasers makes the radiotherapy application of laser generated x-rays an attractive alternative.
U.S. Reissue Pat. No. 34,421 describes a miniature x-ray source for oncological application based on a glass tube incorporating a tungsten anode and a heated thermionic cathode and alternatively a field emission cathode. U.S. Pat. No. 5,737,384 to Fenn describes an apparatus useful for the treatment of tumors in a patient's body by radiotherapy, which utilizes the traditional method of x-ray generation based on acceleration of electrons emitted from a thermionic cathode situated externally to the treated volume. The electrons are accelerated through a thin hollow needle toward the anode placed in the treatment area. U.S. Pat. No. 5,729,583 to Tang et al. relates to a miniature x-ray source for radiotherapy which utilizes several types of field emission, ferroelectric and solid state thermionic cathodes, U.S. Pat. No. 5,428,568 to Oettinger et al. describes an x-ray source with a flexible fiber optical cable, utilizing a light pulse for the generation of an electron pulse from a photocathode located at the end surface of the optical cable. U.S. Pat. No. 5,684,822 describes a miniature x-ray device based on a cold cathode emission electron source.
All above cited patents describe devices for generation of x-rays in energy range from 10 60 keV. While fit for their intended purpose, such devices lack one or more of the following features: the capability of obtaining high fluxes of x-rays, capability to miniaturize devices in size, minimization of the quantity of heat produced during the x-ray generation process and the capability of efficient removal of generated heat.